Ground-Water Quality Related to Irrigation with Imported Surface or Local Ground Water
1974
Nightingale, H. I. | Bianchi, W. C.
Ground-water quality in an arid irrigated area that imports high quality surface water was compared with an adjacent up-gradient area that uses local pumped ground water. A large irrigation canal separates the two areas which are up-gradient from the pumping depression of the Fresno-Clovis, California metropolitan area, which is dependent on ground water. Intensive sampling (154 wells) was done in the fall of 1972 after it was shown by previous sampling that the ground-water electrical conductivity (EC), NO₃⁻, and Cl⁻ content had not significantly changed since 1967. Diagrammatic distribution maps for ground-water quality showed great variability. The Wilcoxon-Mann-Whitney nonparametric statistical tests showed the significance probabilities for EC, NO₃⁻, and Cl⁻ differences above and below the canal were 0.093, 0.114, and < 0.00006, respectively. The significance of these differences are determined by the reader or decision-maker by his chosen error probability, the error risk he is willing to take in rejecting the null hypothesis of no difference when it is true. Areas of higher ground-water NO₃⁻ and Cl⁻ were generally related to soil drainage-recharge and agricultural use. The use of local ground water without supplemental surface supplies above the canal has led to ground-water EC, NO₃⁻, and Cl⁻ concentrations to be 9.5, 18.6, and 91.8% higher, respectively, than below the canal which uses mostly high quality surface water.
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